I. Field
The present invention relates generally to communication, and more specifically to techniques for transmitting data in a wireless communication system.
II. Background
Wireless communication systems are widely deployed to provide various communication services such as voice, packet data, multi-media broadcast, text messaging, and so on. These systems may utilize orthogonal frequency division multiplexing (OFDM), which is a multi-carrier modulation technique that can provide good performance in many wireless environments. OFDM partitions the overall system bandwidth into multiple (S) orthogonal frequency subbands. These subbands are also called tones, subcarriers, bins, and frequency channels. With OFDM, each subband is associated with a respective carrier that may be modulated with data. Up to S modulation symbols may be sent on the S subbands in each OFDM symbol period. Prior to transmission, the modulation symbols are transformed to the time-domain with an S-point inverse fast Fourier transform (IFFT) to generate a transformed symbol that contains S time-domain samples.
A key attribute of OFDM is the ability to combat delay spread, which is a prevalent phenomenon in a terrestrial communication system. The delay spread of a wireless channel is the time span or duration of an impulse response for the wireless channel. This delay spread is also the difference between the earliest and latest arriving signal instances (or multipaths) at a receiver for a signal transmitted via the wireless channel by a transmitter. These signal instances may have traveled via a direct/line-of-sight path and indirect/reflected paths formed by obstructions in the environment. The received signal at the receiver is a superposition of all of the arriving signal instances.
Delay spread causes intersymbol interference (ISI), which is a phenomenon whereby each symbol in the received signal acts as distortion to one or more subsequent symbols in the received signal. The ISI distortion degrades performance by impacting the receiver's ability to correctly detect the received symbols. Delay spread can be conveniently combated with OFDM by repeating a portion of each transformed symbol to form an OFDM symbol. The repeated portion is called a cyclic prefix or a guard interval. The cyclic prefix length is equal to the number of samples that is repeated for each transformed symbol.
The cyclic prefix length determines the amount of delay spread that can be combated with OFDM. A longer cyclic prefix length can combat more delay spread. The cyclic prefix length is typically set based on the maximum expected delay spread for a given percentage (e.g., 95%) of the receivers in the system. Since the cyclic prefix represents overhead for each OFDM symbol, it is desirable to have a cyclic prefix length that is as short as possible in order to reduce overhead.
There is therefore a need in the art for techniques to mitigate the deleterious effects of delay spread while reducing overhead.